Fiber reinforced plastics (FRP) comprising a reinforcing fiber and a matrix resin are widely used in various industrial applications because of their light weight and excellent mechanical properties. Of the FRP, the FRP prepared by using a thermoplastic resin have recently come into the spotlight due to their feasibility of mass production by high speed molding and their recyclability in addition to their light weight and mechanical properties.
Components and structures using an FRP is generally produced by integrating a plurality of members or materials and a step of joining such members or materials is involved in the production of such components or structures. Exemplary known method used for the joining include mechanical joining using bolt, rivet, screw, or the like and the joining using an adhesive. The mechanical joining may suffer from the increased cost due to the processing using joint part, increased weight due to the use of the bolt and the like, brittleness due to concentration of the stress to the processed part, and the like despite its wide applicability. The joining using an adhesive may suffer from the need of the additional step of coating the adhesive, the fact that the limit of the joining strength depends on the strength of the adhesive, insufficient reliability of the joint, and the like.
In the meanwhile, a known method which can be uniquely used in the case of the FRP prepared by using a thermoplastic resin is welding. Since the thermoplastic resin has the nature that it melts by heating and a high-cycle joining at a low cost is possible by using such nature, active technology development on the melt joining is under way. However, such melt joining cannot be conducted in the case of the mutually incompatible thermoplastic resins, and such mutually incompatible thermoplastic resins suffered from the problem of frequent peeling at the interface. In view of such situation, there have been disclosed techniques where a minute anchoring structure is formed at the interface between a FRP prepared by using another thermosetting resin and another FRP prepared by using a thermoplastic resin to thereby improve the bonding between the different resins, and also disclosed are inventions wherein strength of the adhesive layer is defined (Patent Documents 1, 2, 3, 4, 10, and 11). The techniques disclosed in these Patent Documents require use of a thermosetting resin having a low viscosity. In addition, in the techniques disclosed in these Patent Documents, a continuous fiber is used for the thermosetting resin, and molding of the articles having a complicated shape is impossible, and reprocessing was also impossible. Furthermore, adhesion realized by such minute anchoring structure described in these Patent Documents was insufficient for joining mutually incompatible thermoplastic resins.
In the meanwhile, Patent Document 5 discloses a technique wherein a base material comprising a thermoplastic resin and a surface material comprising another thermoplastic resin are integrated by melting. The technique disclosed in Patent Document 5 does not use the reinforcing fiber and the resulting molded article had low strength.
Patent Documents 6 and 9 disclose an integrated composite material wherein a mat member comprising a reinforcing long fiber has different thermoplastic resins impregnated on opposite sides. These Patent Documents are silent about the type of the reinforcing fibers and their dispersion states
Patent Document 8 discloses a technique wherein adhesion with a thermosetting adhesive or cement is improved by preparing a composite substrate having a polyolefin resin impregnated on opposite sides. In this technique disclosed in the Patent Document, the thermoplastic resin used is limited to one type, and adhesion with many types of material using different thermoplastic resins is limited.
Patent Documents 12 and 13 disclose the technique of improving the adhesion between the resins by forming minute anchoring structure at the FRP interface in the sheet comprising the reinforcing fiber which has been opened to substantially single filament state and the resin. In this technique, the sheet is heated to a temperature not lower than the melting point of the resin to thereby raise the fiber that had been caught by the resin, namely to cause the spring back of the fiber in the porous sheet material to thereby form concave-convex shape on the surface. In the technique disclosed in these Patent Documents, the joining of the sheet materials are estimated to have been realized by the intrusion of the thermoplastic resin into the pores of the porous sheet materials. However, the shape and structure of the pores involved in the anchoring are not regulated, and further improvement in the joining properties is required if the growing market demand is to be satisfied. There has also been the problem that the minute anchoring structure described in these Patent Documents could not realize sufficient adhesion strength in the case of mutually incompatible thermoplastic resins.